sgsn: Add support for authentication triplets

This commit add data structures, functions, initialization, and VTY
commands for per subscriber authentication triplets.

The following VTY command is added:

  - update-subscriber imsi IMSI \
    insert auth-triplet <1-5> sres SRES rand RAND kc KC

Note that the triplets are not really used by the SGSN yet.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index cf0af90..1675331 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -36,9 +36,13 @@
 static struct sgsn_subscriber_data *sgsn_subscriber_data_alloc(void *ctx)
 {
 	struct sgsn_subscriber_data *sdata;
+	int idx;
 
 	sdata = talloc_zero(ctx, struct sgsn_subscriber_data);
 
+	for (idx = 0; idx < ARRAY_SIZE(sdata->auth_triplets); idx++)
+	     sdata->auth_triplets[idx].key_seq = GSM_KEY_SEQ_INVAL;
+
 	return sdata;
 }